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Magnesite Enstatite Forsterite CO2 in the Ranges 6–25 Kbar and 700

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Magnesite Enstatite Forsterite CO2 in the Ranges 6–25 Kbar and 700 American Mineralogist, Volume 83, pages 213±219, 1998 Experimental determination of the reaction: Magnesite 1 enstatite 5 forsterite 1 CO2 in the ranges 6±25 kbar and 700±1100 8C ANDREA M. KOZIOL1 AND ROBERT C. NEWTON2 1Department of Geology, University of Dayton, Dayton, Ohio 45469-2364, U.S.A. 2Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, U.S.A. ABSTRACT The P-T equilibrium curve of the reaction of magnesite 1 enstatite 5 forsterite and CO2 was determined in the ranges 6±25 kbar and 700±1100 8C by 53 reversed experiments carried out in 1.91 cm diameter piston-cylinder apparatus with NaCl pressure medium. Silver oxalate was used as a CO2 source and charges were buffered at hematite-magnetite oxygen fugacity. Reaction progress was monitored by weight changes upon puncturing of CO2-in¯ated quenched charge capsules and was con®rmed by comparing X-ray diffrac- tograms of charges made before and after the experiments. The data satisfy the relation: 23 25 2 PEQ 523.215 2 4.784 3 10 T 1 2.542 3 10 T with P in kbar and T in 8C. A P-T curve calculated with the 1994 version 2.3 THERMOCALC program agrees well with our results except in the highest pressure range. Our data, together with three existing tight experimental brackets on the decarbonation reaction of magnesite to periclase and CO2, 0 give DH f (298 K) (forsterite) 5261.20 6 0.87 kJ/mol from the oxides. This value is in agreement with the value adopted by Berman (1988). An analysis of various proposed equations of state for CO2 shows that only those of MaÈder and Berman (1991) and Frost and Wood (1997) meet the constraints imposed by this study near 1000 8C and 10±20 kbar. The modi®ed Redlich-Kwong (MRK)-based equations of state overestimate CO2 fu- gacity in this pressure range. INTRODUCTION This study presents the results of piston-cylinder ex- Carbonate minerals may be the major reservoir for C periments on reaction 1 conducted over a broad temper- in the mantle and therefore may play an important role ature-pressure range, 700 to 1100 8C and 6 to 25 kbar. This range bridges the gap between the previous high- in the global carbon cycle. The system MgO-SiO2-CO2 provides a simple chemical model for understanding the pressure study of Newton and Sharp (1975) and the 2 relationship between silicate, carbonate, and reduced car- kbar work of Johannes (1969). Analysis of our data along bon phases in the mantle. Mg-rich olivine and orthopy- with results on the reaction magnesite 5 periclase 1 CO2 roxene are the major minerals of mantle peridotites, and (Johannes and Metz 1968; Koziol and Newton 1995) magnesite is a stable carbonate at mantle pressures great- leads to an improved value for the enthalpy of formation er than about 3.5 GPa (Brey et al. 1983; Kushiro et al. of forsterite. In addition, because reaction 1 is tightly 1975). Magnesite has been shown to be in equilibrium bracketed by our experiments from 6±25 kbar, we can place limitations on the fugacity of CO2 in this pressure- with (Mg,Fe)SiO3 perovskite and magnesiowuÈstite at 50 GPa and 1500±2500 K (Biellman et al. 1993), and it also temperature range and evaluate several published CO2 has been noted, though rarely, as an inclusion in diamond equations of state. (Wang et al. 1996). The most important equilibrium governing the stability EXPERIMENTAL METHODS of magnesite in the Earth's mantle is Experiments on reaction 1 were made using procedures similar to those described in Koziol and Newton (1995). MgSiO 1 MgCO 5 Mg SiO 1 CO . (1) 33242 The experiments were carried out in two different piston- enstatite magnesite forsterite cylinder apparatus with slightly different designs and ca- Normal upper mantle geothermal gradients lie well within pabilities. All experiments were made under piston-out the stability ®eld of the left-hand assemblage; decarbon- conditions. A press engineered for moderate pressures, ation may occur in mantle regions of abnormally high referred to here as BG, was used for experiments at 6.5 temperatures (Eggler et al. 1979; Newton and Sharp to 15.5 kbar. Calibration experiments showed that no fric- 1975). However, modeling the role of decarbonation re- tional pressure correction is required and that maximum actions in the mantle requires an equation of state for CO2 pressure uncertainties are 6200 bars. Another press, en- that is accurate at mantle pressure-temperature conditions. gineered for higher pressure conditions and referred to 0003±004X/98/0304±0213$05.00 213 214 KOZIOL AND NEWTON: STABILITY OF ENSTATITE AND MAGNESITE TABLE 1. Reversed experiments on the reaction magnesite 1 obtained for the BB press by Jenkins et al. (1985) and enstatite 5 forsterite 1 CO2 (reaction 1) Koziol and Newton (1988). Pressures reported in this pa- Expect- Actual per are nominal pressures. ed CO2 CO2 Reaction Samples consisted of homogenized powders of syn- Expt. Press T P Time loss loss direction thetic enstatite, synthetic magnesite, and synthetic for- no. used (8C) (kbar) (h) (mg) (mg) by XRD sterite, sealed in 1 mm diameter platinum tube segments 5.154 BG 712 6.5 145 1.12 0.84 magn 1 en with weighed amounts of silver oxalate (Ag C O )asthe 5.135 BG 712 7.1 99 1.13 0.56 magn 1 en 2 2 4 MEF-11 BG 720 6.5 168 1.09 1.08 no reaction CO2 source. Synthetic forsterite was prepared from a thor- 5.148 BG 722 7.0 118 1.19 1.01 magn 1 en oughly homogenized stoichiometric mix of ultrapure SiO2 MEF-13 BG 725 6.5 169 0.82 1.01 fo 1 CO2 5.143 BG 732 6.9 119 1.14 1.24 no reaction glass and synthetic magnesite. The mix was decarbonated 5.151 BG 737 6.9 116 1.10 1.13 no reaction in a platinum crucible at 800 8C and then sintered at about MEF-12 BG 740 7.0 266 0.89 1.14 fo 1 CO2 1200 8C for 3 d. The material was ®nely ground, pel- 5.121 BG 742 8.5 88 1.07 0.84 magn en 1 letized, and recycled at approximately 1400 8Cfor2d. 5.122 BG 750 6.7 142 1.16 1.50 fo 1 CO2 5.146* BG 750 8.1 118 Ð Ð magn 1 en The resulting material was 99% forsterite, with a very MEF-5 BG 755 8.0 148 0.84 0.64 magn 1 en small amount of protoenstatite, as shown by optical ex- 5.128 BG 757 8.1 120 0.97 1.12 no reaction MEF-6 BG 760 8.0 144 1.15 1.04 magn 1 en amination and X-ray diffraction. Unit-cell dimensions of MEF-3 BG 765 8.0 150 1.59 1.83 fo 1 CO2 the forsterite determined by X-ray scans with an internal MEF-4 BG 774 8.0 124 1.22 1.61 fo 1 CO 2 standard, are (in angstroms): a 5 4.752(1), b 5 10.200(2), MEF-2 BG 780 8.0 146 0.86 1.09 fo 1 CO2 5.119 BG 782 8.0 96 1.09 1.44 fo 1 CO2 and c 5 5.984(1). Preparation of other materials is dis- MEF-1 BG 800 8.0 165 1.31 1.71 fo 1 CO2 cussed in Koziol and Newton (1995). 5.118 BG 801 10.0 70 1.12 0.82 magn 1 en 5.133 BG 812 10.2 73 1.10 0.77 magn 1 en All experiments were conducted with a hematite-mag- 5.138 BG 815 10.0 119 1.16 0.75 magn 1 en netite buffer consisting of 35±50 mg sintered reagent he- 5.123 BG 822 10.2 120 1.10 0.97 magn 1 en matite and 5±10 mg of H O sealed with the platinum MEF-7 BG 825 10.0 73 1.07 1.21 fo 1 CO 2 2 capsule ina3mmdiameter gold tube segment of 0.15 5.116 BG 832 10.1 73 0.976 1.17 fo 1 CO2 5.120 BG 842 10.0 93 1.17 1.41 fo 1 CO2 mm wall thickness. Magnetite formed from this mixture 5.129 BG 842 10.2 17 1.07 1.08 no reaction very quickly at the temperatures and pressures of the ex- MEF-10 BG 850 12.0 71 0.81 0.41 magn 1 en MEF-9 BG 870 12.0 72 1.17 1.43 fo 1 CO2 periments. Experiments were considered well-buffered if MEF-8 BG 880 12.0 69 1.2 1.46 fo 1 CO2 no weight loss of the gold capsule was detected and if 5.126 BG 914 14.1 16 1.13 1.02 magn 1 en 5.141 BG 923 14.0 68 1.07 0.78 magn 1 en the presence of both magnetite and hematite was con- 5.131 BG 928 14.1 24 1.17 1.02 magn 1 en ®rmed in the buffer product. 5.127 BG 951 14.2 20 1.19 2.03 fo 1 CO 2 Two methods were used to monitor reaction direction. 5.130 BG 955 15.5 14 1.10 0.54 magn 1 en 5.140 BB 963 16.2 7 1.15 0.75 magn 1 en Powder X-ray diffractometer scans of the same charges 5.142 BB 983 16.3 6.5 1.17 0.70 magn 1 en obtained before and after the experiments were compared.
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